This invention relates to means for providing conductive paths from the outer surface of conductor insulating jackets to the magnetic cores in dynamoelectric machines.
In a laminated magnetic core for a dynamoelectric machine the dimensions of the teeth vary somewhat between laminations, and the positions of the laminations vary in the core stack. These irregularities are great enough that the surfaces of the slots have somewhat jagged faces. The coils used in the machine are insulated with outer jackets consisting of wrappings of porous materials inpregnated with certain thermosetting resins and shaped in a mold while the resin is cured to a solid and hard state. This leaves the outer surfaces of the coils very smooth, hard and with some irregularities in their flatness. When these coils are in place in the slots, the smooth outer surfaces of the coils make physical contact with some of the high laminations, leaving voids between the jacket and the remaining laminations.
The variations in the dimensions of the coils also lead to voids or exaggerate the voids mentioned above. If portions of a coil side are smaller than the largest size that the slot will take, the voids left between it and the slot wall will probably be larger and more numerous than they would be with a coil side of this largest size. In other words, a coil side of uniform dimension of this largest size throughout its length will be a closer fit in the slot than a smaller coil side. The actual tolerance of a coil side may be in a range of several mils. Hence the variations in the dimensions of the coils contribute equally or perhaps more to the formation of voids than do the laminations of the core stack.
Electrical grade resinous materials should be good insulators of electricity and reasonably good conductors of heat. Certain epoxy resins meet this specification. However, those that do cure to a hard state, and once fully cured, they do not soften appreciably when reheated during operation of the machine. As a result, the resin impregnants do not soften when the coils become hot and flow into the voids as did the asphaltic impregnants that preceded them. Becuase the resinous materials do not soften with heat and flow into the voids, the voids remain.
Initially, the armor covering on the coils make good electrical contact with the laminations projecting farthest into the slots. These contacts placed the armor and core at essentially the same potential. However, vibration from machine operation will often break these contacts and cause a difference of potential between the armor and core. This potential difference imposes electrical stresses on the air in the voids, stresses that may well be great enough to cause partial discharge from the coil surfaces to the core, i.e., a phenomenon often referred to as corona or corona discharge. The improved resinous materials make higher operating voltages possible, and this in turn subjects the void regions to higher electrical stresses, or these newer insulations may even increase stresses without an increase in voltage. It is well known that in the presence of corona discharge insulating materials are eroded and may eventually break down.
Therefore, the primary object of this invention is to inhibit corona in dynamoelectric machines.
The voids between the coil sides and the cores of dynamoelectric machines also act as barriers to the transfer of heat from the winding to the core laminations. It is well known that the rating of a machine is limited by its ability to get rid of heat, and it is therefore very important that there be optimum heat transfer from the winding to the core.
A further object of the invention is to inhibit corona in combination with improving the transfer of heat from the winding to the core laminations.
According to the invention conductive paths are provided between the winding coils and the core laminations of a dynamoelectric machine by placing a semiconductive elastomeric material between the coil sides and the walls of the core slots. The elastomeric material is of a type which will deform sufficiently under pressure to flow into the irregularities between the coil side and the slot walls, and in so doing provide condutive paths from the outer jackets of insulation on the coils to the core laminations. This material must be capable of retaining its strength, elasticity, conductivity, etc., and remaining in place between the coils and core under vibration, coolant flow, electric stresses, repeated temperature changes, etc., for the operating life of the machine.
Certain silicone resins are well suited for use as conductive path forming materials between the coils and core. Inherently, silicone resins are good electrical insulators, but some are relatively good conductors of heat as well. The good heat conductors are preferred because they will transfer heat from the coils to the core. However, to make them electrically conductive for corona inhibiting purposes according to the invention, they are filled with conductive fine particle materials such as carbon powder, lamp black or a mixture thereof. The amount of conductive powder added to the resin is just enough to give it the required electrical properties, but not enough to detract significantly from the physical properties mentioned in the previous paragraph.
Canadian Pat. No. 932,013, issued Aug. 14, 1973, Lonseth et al discloses a heat conductive material placed between the coil sides and slot walls for transferring heat from the outer surfaces of the coils to the core. The preferred embodiment of this invention may be considered an improvement on the patent in that the material is electrically conductive as well. However, although thermal conductivity is preferred, it is not essential in the practice of this invention.